California Gov. Edmund G Brown Jr., front left, rides in a driverless car to a bill signing at Google headquarters in Mountain View, Calif., Tuesday, Sept. 25, 2012. The legislation will open the way for driverless cars in the state. Google, which has been developing autonomous car technology and lobbying for the legislation has a fleet of driverless cars that has logged more than 300,000 miles (482,780 kilometers) of self-driving on California roads. (AP Photo/Eric Risberg)

U.S. auto safety regulators outlined plans on Thursday to analyze the safety risks and benefits of self-driving cars over the next four years. The National Highway Traffic Safety Administration urged states to bar operation of autonomous cars for purposes other than testing. But if states allow commercial use of these vehicles, drivers should undergo additional training and obtain special licenses, the agency said.

The recommendations were part of NHTSA’s preliminary policy on autonomous vehicles. Such cars can be operated without direct driver input to control steering, acceleration and braking, the policy states.

A study by J.D. Power and Associates in late April showed that consumers were warming to the idea of autonomous vehicles, but preferred features like automatic park assistance or emergency braking to a fully autonomous mode.

Most automakers, including General Motors Co, Ford Motor Co and Toyota Motor Corp, feature vehicles with some form of automation.

The NHTSA policy defined four types of self-driving cars. On the low end are cars with features like adaptive cruise control that allow the car to adjust its speed to maintain a safe distance from the surrounding vehicles.

On the top end of the scale are vehicles designed to perform “all safety-critical” driving functions and monitor roadway conditions during a trip, the policy states.

NHTSA will now research the safety risks of vehicles with higher levels of automation. The agency will study how the driver interacts with such cars as well as ways to make the car’s electronic control systems more secure and reliable.

“Self-driving vehicle technology is not yet at the stage of sophistication or demonstrated safety capability that it should be authorized for use by members of the public for general driving purposes,” the agency said in its report.

But should a state decide to permit self-driving cars outside of testing, it should require drivers obtain a special license, NHTSA said. Drivers should be able to demonstrate that they have driven a self-driving car for a “certain minimum number of hours.”

As news of the deadly mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, unfolded last week, Pia Guerra, a 46-year-old Vancouver-based artist, felt helpless. She couldn’t bring herself to go to sleep, so she began to draw.

Police who find suspected drugs during a traffic stop or an arrest usually pause to perform a simple task: They place some of the material in a vial filled with liquid. If the liquid turns a certain color, it’s supposed to confirm the presence of cocaine, heroin or other narcotics.